1. Field of the Invention
The present invention relates to a fuel injection system for an internal combustion engine, which system has an air supply passage for supplying air to a point close to the fuel injection hole of a fuel injector to realize good atomization of the injected fuel.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 4-295178 discloses such a fuel injection system, in which the air supply passage has a control valve and is connected to the intake passage upstream of the throttle valve. The air supply passage supplies air to a point close to the fuel injection hole of a fuel injector using the differential pressure between the upstream and downstream sides of the throttle valve in the intake passage. In such differential pressure type air supply passage, the amount of air passing therethrough is controlled by the control valve such that a desired amount of intake air in each engine operating condition is collectively supplied into the cylinder, in addition to the amount of intake air passing through the intake passage.
The intake passage has a relatively large cross section such that a large amount of intake air, during a high engine load period, can pass therethrough. On the other hand, in the air supply passage, the downstream side of the control valve must have a relatively small cross section to realize a speed-up of the air-stream therethrough. Accordingly, when the degree of opening of the throttle valve is rapidly increased, in a sharp acceleration, the pressure in the intake passage downstream of the throttle valve becomes close to atmospheric pressure. At this time, even if the degree of opening of the control valve is rapidly increased, the pressure in the air supply passage downstream of the control valve remains negative for some time. Whereby, the pressure in the intake passage downstream of the throttle valve is higher than in the air supply passage and thus a back-flow of intake air occurs, from the intake passage downstream of the throttle valve to the air supply passage, so that some of the fuel already supplied into the intake passage flows into the air supply passage with the back-flow of intake air. Thus, a desired amount of fuel is not supplied into the cylinder and the air-fuel ratio of the mixture becomes more lean than a desired air-fuel ratio so that a desired engine torque can not be obtained and a purification of exhaust gas in the three-way catalytic converter is deteriorated by the lean mixture.